1. Field of the Invention
The present invention relates to the field of circuits providing a dual power supply from an A.C. voltage. An example application of the present invention is the generation of supply voltages of control blocks of a power variator intended to be series-connected with a load to be supplied by an A.C. voltage (generally the mains voltage), and based on two switches which are bi-directional for the voltage and unidirectional for the current, connected in anti-parallel.
2. Description of the Related Art
FIG. 1 schematically shows a conventional example of circuit 1 for controlling the power variation of a load Q supplied by an A.C. voltage Vac. Two switches K1 and K2, formed, for example, of IGBT transistors with a reverse voltage hold, are connected in anti-parallel between two terminals 2 and 3 of the power variator. The variator is series-connected with load Q between two terminals P and N of application of voltage Vac. Each transistor K1 or K2 has its gate connected to the output of a circuit (circuits 4 (DRV1) and 5 (DRV2) respectively), generating an appropriate control signal based on a power reference. The power variation is performed with a phase angle and consists of controlling the turn-on (or turn-off) time of transistors K1 and K2 according to the considered halfwave of the A.C. voltage. The power setting is performed by circuits 4 and 5 which, for example, receive a reference signal CTRL. In the example of FIG. 1, supply voltages V1 and V2 are respectively supplied by circuits 6 (ALIM1) and 7 (ALIM2) extracting D.C. voltages V1 and V2 respectively from the voltage between terminals 2 and 3, and between terminals 3 and 2 respectively.
Other power switches K1 and K2 may be used. For example, these may be MOS or IGBT transistors, each in series with a diode, or thyristors.
A recurrent problem of this type of circuit is to provide control circuits 4 and 5 with supply voltages V1 and V2 as well as with a common control reference, because circuits 4 and 5 are not referenced to the same potential (the potential of node 3 is used for circuit 4 and the potential of node 2 is used for circuit 5).
FIG. 2 shows a conventional example of an assembly for providing supply voltages V1 and V2 at the same time as a common control signal for the two circuits 4 and 5. For simplification, transistors K1 and K2 and the load have not been shown in FIG. 2.
Each supply circuit 6, 7 is formed of a capacitor C1, C2 respectively, across which is sampled voltage V1, V2, respectively. Capacitor C1 or C2 is in series with a diode D1, D2, respectively, and this series association is connected in parallel with a zener diode DZ1, DZ2, respectively, setting the value of voltage V1, respectively V2. Supply Circuits 6 and 7 are, in this example, connected to each other by a potentiometer P intended to set the power reference desired for the load. The power variation is then performed by setting the time of provision of the corresponding supply voltage to circuits 4 and 5 with respect to the zero crossings of voltage Vac. In positive halfwaves (positive voltage Vac between terminals P and N, FIG. 1), voltage Vdim across terminals 2 and 3 of the power variator is also positive. A current then flows through forward-biased diode DZ2, through potentiometer P, then through forward-biased diode D1, to charge capacitor C1. The value of potentiometer P conditions the charge speed of capacitor C1. Diode DZ1 sets, by its threshold voltage, voltage V1. In negative halfwaves, a current flows in diode DZ1, then in potentiometer P, then into diode D2, to charge capacitor C2. The value of potentiometer P conditions the charge speed of capacitor C2. Diode DZ2 sets, by its threshold voltage, voltage V2.
Assemblies such as those of FIGS. 1 and 2 are described, for example, in European patent number EP1,416,620 of the applicant.
FIG. 3 shows another conventional example of an assembly for providing D.C. supply voltages V1 and V2 of opposite polarities from an A.C. voltage. As applied to a power variator, the assembly of FIG. 3 provides voltages V1 and V2 to circuits 4 and 5 of FIG. 1. Starting from the assembly of FIG. 2, potentiometer P is replaced with an active control circuit 8. Circuit 8 is formed of a MOS transistor M forming a variable current source, linearly controllable by an electronic circuit 9 (CT), and interposed between the two circuits 6 and 7 of provision of supply voltages V1 and V2. To enable an operation by means of a single MOS transistor, four diodes D3, D4, D5, and D6 are assembled in a fullwave bridge between the respective anodes of diodes D1 and D2 (or the respective cathodes of zener diodes DZ1 and DZ2). For example, the anode of diode D3 and the cathode of diode D5 are connected to the anode of diode D2, while the anode of diode D4 and the cathode of diode D6 are connected to the anode of diode D1. The cathodes of diodes D3 and D4 and the anodes of diodes D5 and D6 are respectively connected to supply terminals 10 and 11 of circuit 9. MOS transistor M forming the variable current source is, in practice, in series with a resistor R between terminals 10 and 11. Circuit 9 typically comprises a resistor R9 between the drain and the gate of transistor M and a zener diode DZ9 between the gate of transistor M and terminal 11.
An active supply circuit such as described in relation with FIG. 3 enables reduction of losses when the load is in stand-by and needs not be supplied, since circuit 9 can then control the turning-off of transistor M. However, this requires modifying circuit 9 so that it exploits information about voltages V1 and V2. Now, the provision of such information poses a reference problem, since circuit 9 has no common reference with voltages V1 and V2. It could be devised to use two opto-couplers to transmit this information to circuit 9. An obvious disadvantage however is the cost and bulk.